Antigenic and membrane changes in trypanosomes are critical in the relationship between these parasites and their hosts. The significance of these changes in immune escape and in the developmental biology of trypanosomes has yet to be clarified. We have previously demonstrated that membrane permeability is altered during infection and during the developmental cycle of trypanosomes. Phosphorylation of specific membrane proteins by cyclic nucleotide-dependent protein kinases is thought to affect membrane functions in other cell systems. We have determined that intracellular concentrations of cyclic AMP are lower in reproducing than in nonreproducing forms of T. lewisi. More recently we have monitored parasitemia levels and measured intracellular cyclic AMP concentrations in a triple-cloned, pleomorphic strain of T. b. brucei. Concentrations of this nucleotide are low, 25 pmol/10 to the 9th power slender trypanosomes, during the initial phase of infection in rats. Concentrations increase to 60 or 70 pmol 10 to the 9th power organisms as the parasitemia peaks, then decrease as the population shifts to intermediate and stumpy forms. They drop back to the original low levels when the parasites assume a predominantly stumpy appearance. We have also demonstrated the presence of agenyl cyclase activity and endogenous protein kinase and acceptor protein activity in plasma membrane-enriched fractions prepared from trypanosomes. In the proposed studies we will examine the effect of changes in cyclic AMP levels on growth, reproduction, membrane structure and permeability in trypanosomes. We will use monospecific antibodies and antibody fragments, and agents known to alter intracellular cyclic AMP levels in order to determine the role of cyclic AMP in regulating membrane changes. A combination of cell fractionation and protein purification will be used to focus on the significance of membrane phosphorylation during some of the complex changes trypanosomes undergo in the mammalian host and in culture.